Fill level measuring devices working according to the travel time principle are applied in a large number of branches of industry, e.g. in various types of processing, in the chemicals industry or in the foods industry.
Typically, the fill-level measuring device is mounted above the container. In measurement operation, in each measuring cycle, a transmission signal, e.g. a microwave signal, is sent by the fill-level measuring device toward the fill substance in the container and its fractions reflected back in the container to the fill-level measuring device received as a received signal according to a travel time dependent on its path traveled in the container. Based on the received signals, echo functions are derived, which give the amplitudes of the received signals as a function of a position corresponding to their travel time or their path traveled in the container. In such case, travel time and path length are convertible into one another based on the propagation velocities of the transmitted and received signals along the traveled paths.
Reflections from reflectors located in the container, such as e.g. the surface of the fill substance, a disturbance located in the container and the container floor, bring about in the echo functions local maxima, referred to herein as echos, at echo positions in the echo function corresponding to distances of the reflectors from the fill-level measuring device.
Applied for determining the travel times can be all known methods, which enable measurement of relatively short distances by means of reflected signals. For microwave signals, these are usually the pulse radar and the frequency modulation continuous wave radar (FMCW radar) methods.
In the case of pulse radar, in each measuring cycle, short microwave transmission pulses are periodically emitted, which are reflected in the container and received back according to the travel time dependent on the path length traveled.
In the case of the FMCW method, in each measuring cycle, a microwave signal is sent continuously, which is periodically linearly frequency modulated, for example, according to a sawtooth function. The frequency of the received signal has, consequently, compared with the instantaneous frequency of the transmission signal at the point in time of receipt of the reflected signal, a frequency difference, which depends on the travel time of the associated microwave signal. The frequency difference between transmission signal and received signal, which can be won by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, corresponds, thus, to the travel time and therewith to the separation of the reflecting surface from the fill-level measuring device. Furthermore, the amplitudes of the spectral lines of the frequency spectrum won from the Fourier transformation correspond to the echo amplitudes. This Fourier spectrum thus represents, in this case, the echo function.
Then, based on predetermined echo recognition methods, wanted echos of predetermined wanted echo types are ascertained in the echo functions. Each wanted echo is a local maximum of the echo function occurring at an associated echo position and is attributable to a reflection on a certain predetermined reflector in the container, especially on the surface of the fill substance, on the container floor or on a disturbance located in the container. Wanted echos are distinguished according to wanted echo type based on the associated, determined reflectors. The most important wanted echo for fill level measurement is the fill-level echo of the reflection on the surface of the fill substance. Other wanted echo types relevant for fill level measurement are the container floor echo attributable to reflections on the container floor, and disturbance echoes attributable to reflections on previously known disturbances.
Echo recognition methods for identification of wanted echos contained in an echo function and methods for checking the plausibility of the associating of an echo to a certain wanted echo type are described, for example, in now published, German patent application DE 10 2004 052 110 A1.
Known from fill level measuring technology are a number of different methods, with which the fill level can be determined based on information, especially echo position information, derived from at least one wanted echo.
To the extent that a fill-level echo can be determined in the echo function, fill level is always determined based on the echo position of this fill-level echo. The echo position corresponds to the travel time, which the transmission signal requires for traveling the path from the transmitting and receiving system to the surface of the fill substance and back. From this travel time, there results in the case of the known propagation velocity of the transmission signals, directly the separation between the surface of the fill substance and the transmitting and receiving system, which can then, based on an installed height of the transmitting and receiving system, be directly converted into the associated fill level.
In cases in which no fill-level echo can be determined, or the classification of an echo candidate for fill-level echo appears questionable, frequently alternative methods are applied for fill level determination, in the case of which supplemental information derived from wanted echos of other wanted echo types is considered.
Thus, for example, fill level can be determined based on the propagation velocity of the signals in the fill substance and the echo position of the container floor echo. If the fill level lies above the installed height of a disturbance, then it can be determined based on the propagation velocity of the signals in the fill substance and the echo position of the associated disturbance echo.
Likewise, a fill-level echo contained in the echo function but not directly identifiable as such can be ascertained based on supplemental information derived from the wanted echos of other wanted echo types. Thus, for example, the echo position of the fill-level echo can be calculated based on the echo position of a container floor echo identified in the echo function, and that local maximum of the echo function subsequently identified as fill-level echo, whose echo position lies nearest the fill level-echo position calculated from the echo position of the container floor echo.
Moreover, methods are known, in the case of which, based on earlier recorded echo functions, a time development of the echo position of the wanted echo of the respective wanted echo type is determined, and therefrom the current echo position of the respective wanted echo is extrapolated.
In the case of all of these methods, there is the problem that information derived from the wanted echos is not always equally reliable.
A frequently occurring cause for this involves, as a rule, not calculable, or not predictable, signal interferences occurring in the container, in the case of which the reflected signal components superimpose in the container with one another or with the transmission signal constructively or destructively. This can lead to a splintering of the echo function in the region of the wanted echo, in the case of which in the immediate vicinity of the local maximum identified as wanted echo other local maxima of comparable amplitude occur. In the case of destructive interferences, it can even happen that the echo function has at the desired position, where the wanted echo would actually occur due to the distance of the associated reflector from the transmitting and receiving system, a local minimum. This can lead to the fact that a local maximum of the echo function directly neighboring this desired position is determined to be the wanted echo. Such an incorrect determination remains frequently unnoticed, and can, as a rule, also not be discovered based on plausibility checks, even with the aid of information derived from additional wanted echos.
Accordingly, the application of these wanted echos, respectively information derived therefrom, such as e.g. their echo positions, can lead to a worsening of the accuracy of measurement of the fill level measurement.